Published: February 03, 2011
Napa Vintners Release Findings of Climate Study
ST. HELENA, Calif. - (BUSINESS WIRE) - The Napa Valley Vintners (NVV) non-profit trade association announced
today the release of the Napa Valley-specific climate study titled Climate
and Phenology in Napa Valley: A Compilation and Analysis of Historical
Data by Dr. Daniel R. Cayan, Dr. Kimberly Nicholas, Mary Tyree, and
Dr. Michael Dettinger.
In 2006, a researcher garnered national media attention by predicting
that Napa Valley would soon become too warm to grow fine wine grapes.
These reports noted signs of warming in California and the western
United States in recent decades, calling attention to several changing
indicators in weather, hydrological and biological systems. Evidence
from other Mediterranean climate regions around the world indicated that
climate warming may be taking hold in these settings. However, the
experience of Napa Valley growers has been contrary to the notion that
Napa Valley has warmed substantially. A problem in applying this
previous research to the Napa Valley is that it has considered just a
few weather station records in Napa Valley, which has long been known
for very diverse micro-climates and growing conditions.
This just-released Napa-specific study by Cayan and colleagues
scrutinized weather and phenology (the growing cycle of grapevines)
records based on many more stations within Napa Valley, and arrived at a
number of important new conclusions. Over the four years of the study,
more than 12,000 data points were collected from measurements made at
geographically diverse sites in the valley, using information ranging
from hand-written journals kept by long-time growers to digital data
from current-day automated weather stations positioned valley-wide. Most
of the observations were from records taken since the late 1970s, but
some of the hand-written entries were from as early as the 1950s. The
Executive Summary of the study is attached; in brief, it finds that the
region has experienced some warming, approximately 1° to 2° Fahrenheit
over the past several decades, but considerably less warming than would
be inferred from the standard cooperative observer weather stations in
Napa Valley. The warming has been primarily in winter, spring and
summer, and it has concentrated during nighttime rather than daytime.
Over the last several decades in growing season temperatures, there has
been little warming in the daytime and the available observations
provide little evidence that the growing cycle of the grapevines has
changed substantially.
The results, overall, provide good short-term news that consumers are
not "tasting" climate change in Napa Valley wines. It reinforces the
firmly held belief among growers and winemakers that the taste profile
of Napa Valley's wines is driven by its place of origin, as well as by
the solid direction of the in-field practices related to viticulture
(clonal and rootstock selection, canopy management, irrigation, crop
load and hang time, among others) along with stylistic preferences in
winemaking.
The Napa Valley-specific climate study began in 2006 when the NVV board
of directors created a climate study task force of vintner members who
had both interest and knowledge of the issue. The task force identified
Dr. Dan Cayan of Scripps Institution of Oceanography at UC San Diego,
one of the most experienced climate scientists in the state, to lead a
research team. A key member the team was Dr. Kimberly Nicholas, a North
Coast native, who at the time was in the midst of her doctoral degree
program at Stanford University, studying the potential effects of
climate change on high-quality winegrowing in Napa and Sonoma. With this
team in place, and working with the vintners and growers in Napa Valley,
a set of criteria was designed to collect and investigate as much
historical, in-field data as possible to determine climate patterns and
trends specific to the Napa Valley appellation.
Vintner and climate study task force member Christopher Howell of Cain
Vineyard & Winery said, "We winemakers are farmers--as farmers, we live
not by the climate, but much more by the weather, i.e.: day to day, week
to week, season to season, and year to year. In order to get clear
evidence of climate change, we need to be able to compare trends over
decades--this is not a perspective on the usual human scale." Indeed,
this study and previous research shows that Napa temperatures are
correlated, to some extent, with changing ocean temperatures along the
Pacific coast; for example, sea surface temperatures along the central
California coast have been unusually cool in recent years, associated
with relatively cool air temperatures in the Napa Valley.
Howell continued, "We love the quote attributed to Mark Twain who said,
'The coldest winter I ever spent was a summer in San Francisco.' The
Pacific Ocean is our region's greatest temperature control. From living
here, we know that the warmer the Central Valley becomes on a summer
day, the more intensely the fog pours in from the coast. This is the
'vacuum effect' of the warmer interior valley. We have been blessed to
have the perfect mix of warm days and fog/coastal cooling that allow us
to grow some of the finest wines in the world. Globally, the years 1998,
2005, 2006 and now 2010 were the warmest years on record, but they were
some of the coolest for the Napa Valley. There is a suggestion by some
climate scientists that, as the interior areas warm in the future, Napa
temperatures may actually remain relatively moderate, or even cool as
maritime air gets drawn further up the Valley. Either way, warmer or
cooler, it's different than what we're experiencing today--so as prudent
farmers we need to look at all of our possible scenarios and consider
best practices to continue to grow the best wine grapes," Howell
concluded.
The new study emphasizes the need for maintaining regular observations
at high-quality weather stations around the Valley. Estimates of
temperature changes in the Napa region are hampered by local changes in
exposure, buildings and paved areas around the longest existing weather
stations. For example, the commonly used weather station at Napa State
Hospital, with a record going back 100 years, is situated over an
irrigated lawn next to a black top driveway and a building with a large
window air conditioning unit, and the St. Helena weather station is
mounted on the roof of the city fire station, but was moved three times
in recent decades. Shorter records collected by Napa weather observers
indicate that these long-term stations are registering an artificial
warm bias which has likely increased over the last several decades. The
study recommends that the Napa Valley farming community should formally
assess the adequacy of its current climate observations and establish a
protocol to maintain a high-quality, long-term climate monitoring
network.
"I am proud of the leadership taken by the Napa Valley Vintners in this
climate study. We have strong benchmarks in place that will further
allow us to track what changes may occur in our unique climate--really
specific to the Napa Valley. Though we are just 4% of California's wine
grape harvest, we account for 34% of the value of the California wine
industry on the US economy. It's in all of our best interests to ensure
a long and healthy future in fine wine from the Napa Valley," said
Kathleen Heitz Myers, president of the NVV board of directors.
Additional research beyond this study continues on a number of fronts,
looking at what in-field practices could be employed should climate
change take the form of regional warming, such as how canopies and cover
crops are managed as easy, short-term solutions. Napa Valley growers and
vintners are raising awareness of what can be done locally while
thinking globally with programs like Napa Green Certified Land and
Winery, which are the most comprehensive green initiatives in the wine
industry and that have the well-earned reputation for going above and
beyond when it comes to environmental best practices.
The Napa Valley Vintners is the non-profit trade association responsible
for promoting and protecting the Napa Valley appellation as the premier
winegrowing region. From seven founding members in 1944, today the
association represents 400 Napa Valley wineries and collectively is a
leader in the world-wide wine industry. To learn more about our region
and its legendary American wines, visit www.napavintners.com
Climate and Phenology in Napa Valley:
A Compilation and
Analysis of Historical Data
February 3, 2011
Daniel R. Cayan, Dr. Kimberly Nicholas, Mary Tyree, Dr. Michael Dettinger
Executive Summary
In response to, and sponsored by the Napa Valley Vintners (NVV), this
study was designed to evaluate the climate of the Napa Valley (NV) and
explore links between climate and wine grape phenology and composition.
Specific objectives of this study are to a) describe the spatial and
temporal structure of climate in NV; b) explore linkages between
climate and phenology/harvest; and c) evaluate historical trends
in temperature.
NV shares traits of both coastal and interior climates. The southern
portion of NV is more coast-like while the central northeastern side of
NV is more interior-like. Elevation also plays a strong role, e.g.,
heavier precipitation and lower summer daytime temperatures generally
occur in higher elevations relative to the valley floor.
Approximately 30 private weather stations distributed throughout NV
allow inspection of fine-scale temperature variability from north to
south, and along a transect of changing elevations across the valley
from east to west. These records vary in length, but mostly cover 2 to
15 years from the last two decades. Generally, all stations display
similar variations (i.e., warm periods are relatively warm at all
stations). There is marked variability by location (i.e., southerly and
higher-elevation sites are cooler in summer months, containing lower
daytime maximum temperatures and higher nighttime minimum temperatures
than more inland and valley floor sites. The daily and seasonal
temperature ranges and temperature extremes observed in NV are greater
than cool coastal climates, but smaller than warmer inland climates.
During summer, cloud cover in NV shares patterns more similar to the
coast than to the interior Central Valley. Cloud cover in the southern
part of the NV is greater in July and August than it is in April, May,
and June. Days with higher morning cloud cover in NV generally do not
attain afternoon temperatures as high as those with lower morning cloud
cover, even though cloudiness may dissipate by early afternoon.
In addition to local influences, climate in NV has a very clear,
significant association with large-scale variations and trends.
Large-scale atmospheric circulation plays a strong role in setting up
anomalously warm and anomalously cool days in NV. Anomalous Pacific
Ocean temperature patterns are linked quite strongly to NV temperatures
during winter and early spring months, but not so much during summer
months.
Temperature records from the longer-term stations in the region indicate
that the NV has experienced warming over the last several decades. At
several of the stations, the warming that is detected is stronger during
the nighttime than in the daytime, and it has occurred preferentially
during the year--primarily during January through August. Relatively
high rates of warming in NV are found in 6-9 decades of temperature
records within the NV from the Napa State Hospital and St. Helena
cooperative observer (COOP) stations. Similar warming trends are found
at other cooperative stations surrounding NV.
The trends of minimum and maximum temperature at the COOP (COOP)
stations, which amount to a warming of mean temperature that is
approximately .03 °F/yr since 1931, are essentially the same trends that
have been reported in previous studies of regional temperature in the
Napa region, including the study by Jones and Goodrich (2008). However,
COOP stations have undergone several location changes and have had
instrument changes. The present installations and locations do not give
confidence for stable, un-altered temperature records. The Napa State
Hospital installation is very close to a building and an air conditioner
outlet. St Helena is mounted on the roof of a building. That there may
be an excessively high warming in the Napa and St Helena COOP records is
suggested by comparisons between the temperatures from the COOP stations
with those from other stations in the region. These include the
temperature recorded at and above the earth's surface by the Oakland
radiosonde (upper air sounding) record. California Irrigation Management
Information System (CIMIS) weather records and several sets of vineyard
temperature records mimic the variability shown by the longer records.
The U.S. Historical Climate Network (HCN) cooperative station
temperature record from the National Climatic Data Center provides a
record that is adjusted, in attempt to eliminate spurious trends. The
adjusted trends in maximum and minimum temperature are lower than the
trends from the COOP data, however this change is problematic because of
the lack of a long stable record in the immediate region that could be
used as a reference series. Petaluma, which appears to be the record
having minimal amounts of adjustment, has itself gone through moves and
is in a dubious site as it is also close to a structure. Thus it is
possible that even adjusted Napa State Hospital COOP trend may still be
affected by some, unknown amount of artificial trend, although the
amount of that error could be by way of either too little or too much
warming trend.
While the records that have several decades of record are not of
sufficiently high quality to precisely determine the temperature trend
in the non-developed portions of NV, the evidence suggests that the
warming in most non-urban parts of NV over the last 6-8 decades has been significantly
less than the approximately +0.03°F/year trend in mean temperature
that is contained in the unadjusted COOP stations from Napa State
Hospital and St. Helena. Comparisons between the COOP stations and other
temperature records from sites that are less affected by human
alterations suggests that the amount of nighttime warming has been
significantly less than the .05°F/yr that is derived from the raw COOP
records, and that daytime temperature warming has been close to zero. It
is important to emphasize that while the trends from the COOP stations
appear to be artificially affected and too much warming, there
nonetheless has been a real warming trend. Overall, it appears the
warming that has occurred in the Napa region has mostly occurred during
nighttime hours, as exhibited by daily minimum temperatures. Warming of
nighttime temperature exceeds that of daytime temperature, as indicated
by daily maximum temperatures.
NV grape phenological stage timing and harvest characteristics are
significantly influenced by antecedent weather and climate, with these
antecedent influences in some cases being detected as early as early
winter. A rule of thumb is that earlier phenological stages are linked
to prior warm conditions. Correlations between phenological stages are
relatively robust amongst three stages (bloom, veraison, and harvest),
but weak for any of these stages in association with budburst.
Phenological dates are not very well-correlated with total annual
growing degree-days (January-December), in the sense that higher degree
days associates with earlier phenological timing. However, each
phenological stage is strongly correlated with the accumulation of
specific thresholds of degree-days (which may vary by variety). These
could be used to predict the timing of development by variety.
Analysis of comprehensive county-wide crush reports from 1990 onward
demonstrated that there is a strong trend of increasing Brix at harvest
over time. The most dramatic increase was for Zinfandel, from close to
20 to above 26 over the 18 year record. Sauvignon blanc has seen the
least change, an increase of about 1 degree Brix. Cabernet has increased
from about 23 to 26 degrees. There is also a trend across all varieties
except Sauvignon blanc for decreased yields over time, most dramatically
for Zinfandel (from close to 6 to about 2.5 tons/acre) and Merlot.
Chardonnay yields have declined only slightly, holding steady near 4
tons/acre. Sauvignon blanc has the highest yields of all varieties
studied, and has been increasing over time. There is some synchronicity
in yields across varieties within a given vintage, presumably due to
climatic conditions (for example, 1997 was a high yielding year for all
varieties). Vineyard management practices, used to achieve desired wine
styles, have changed over the last 20 years, in ways that may affect
phenological dates. For example, later pruning can lead to later
budburst dates. Practices such as leaf thinning and cluster thinning may
act to speed up ripening, while vineyard practices such as hedging may
delay ripening. However, we lack access to management records that would
allow examination of the statistical importance of these practices.
Nonetheless, climate plays a dominant role in setting phenological dates
(for example, in initiating bud growth in the spring, and affecting
fruitfulness and berry set, among other factors). As a general pattern,
Brix increases with later harvest dates. However, recent harvests (last
8 years) have been very high in Brix, and not anomalously late.
Generally, the trend has been toward later harvest dates, though it
varies by variety (Pinot earlier; Cab Franc, Cab Sauvignon and Merlot
about 2-3 weeks later over 30+ year period). Recent Brix increase is
believed to be due to stylistic and winemaking preferences, but needs
further investigation.
This data collection and synthesis was a substantial effort on the part
of both contributors and analysts. Now that it has been completed, there
is a valuable template for moving forward. The effort highlights the
need to continue to, and improve monitoring, which will provide the
means for an ongoing assessment of both climate and vine development in
NV.
Napa Valley Vintners
Terry Hall, 707-968-4217
thall@napavintners.com
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